The invention relates to a dispersion managed optical waveguide fiber in which the total dispersion of the fiber is managed by varying the zero dispersion wavelength, .lambda..sub.o, along the length of the waveguide. In particular, optimum performance of the dispersion managed waveguide is found for the case where the high and low ranges of .lambda..sub.o are widely spaced and are asymmetrical relative to the operating wavelength.
The introduction into the telecommunications network of high powered lasers, optical amplifiers, multiple channel communications, and higher bit rates has resulted in the exploration of waveguide fiber designs which can minimize signal degradation due to non-linear waveguide effects.
Of particular interest is a waveguide design which can substantially eliminate four wave mixing. A dilemma arises in the design of a waveguide fiber to minimize four wave mixing while maintaining characteristics required for systems which have long spacing between regenerators. That is, in order to substantially eliminate four wave mixing, the waveguide fiber should not be operated near its zero of total dispersion, because four wave mixing occurs when waveguide dispersion is low, i.e., less than about 0.5 ps/nm-km. On the other hand, signals, having a wavelength away from the zero of total dispersion of the waveguide, are degraded because of the non-zero total dispersion. One strategy proposed to overcome this dilemma, is to construct a system using cabled waveguide fiber lengths some of which have a positive total dispersion and some of which have a negative total dispersion. If the length weighted average of dispersion for all the cable segments is close to zero, the regenerator spacing can be large. However, the signal essentially never passes through a waveguide length where the dispersion is close to zero, so that four wave mixing is substantially reduced.
The problem with this strategy is that each link between regenerators must be tailored to give the required length weighted average of dispersion. Maintaining cable dispersion identity from cabling plant through to installation is an undesirable added task and source of error. Further, the need to provide not only the proper dispersion, but also the proper length of cable having that dispersion, increases the difficulty of manufacture and leads to increased system cost. Another problem arises when one considers the random lengths and dispersions which might be needed for replacement cables.
The present invention overcomes these problems by making each individual fiber a self contained dispersion managed system. A specified, i.e., pre-selected, length weighted average of total dispersion, i.e., total dispersion product, is designed into each waveguide fiber. Thus, the cabled waveguide fibers all have essentially identical dispersion product characteristics and there is no need to assign a particular set of cables to a particular part of the system.
Power penalty due to four wave mixing is essentially eliminated, or reduced to a pre-selected level, while total link dispersion is held to a pre-selected value, which may be a value substantially equal to zero.
The waveguide having such a length weighted dispersion characteristic and a method for making the waveguide are described in U.S. patent application Ser. No. 08/423,656.
However, even for the dispersion managed waveguide fiber of the cited application, a power penalty for four wave mixing is sufficiently large to limit communication systems which operate at state of the art rates and distances or which feature closely spaced wavelength division multiplexed channels.